Pores in III–V Semiconductors

The paper reviews electrochemically etched pores in III–V compound semiconductors (GaP, InP, GaAs) with emphasis on nucleation and formation mechanisms, pore geometries and morphologies, and to several instances of self‐organization. Self‐ organization issues include the formation of single‐crystalline two‐dimensional hexagonal arrays of pores with lattice constants as small as 100 nm found in InP, synchronized and unsynchronized diameter oscillations coupled to current and voltage oscillations, and pore domain formation. The findings are discussed in relation to pores observed in silicon. Some novel properties of the porous layers obtained in III–V compounds are briefly described.     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Organometallic molecular precursors for low-temperature MOCVD of III–V semiconductors

Metal-organic chemical vapor deposition (MOCVD) is a suitable technique for the preparation of III–V epitaxial layers which are used in the fabrication of microelectronic and optoelectronic devices. The usual Ga and As sources for GaAs are Ga(CH₃)₃ and AsH₃, respectively. However, the use of these precursors has some disadvantages including: The toxicity and storage of arsine, stoichiometry control, carbon incorporation and unwanted side reactions. Several groups of researchers have investigated alternative sources for both the group–III and group-V elements. A review of these new organometallic precursors is presented in this paper. However, because group-III and group-V elements form Lewis-acid/base adducts in the MOCVD reactor, we have especially investigated the use of this class of compounds as single starting molecules. Several adducts have been successfully used for the epitaxial growth of GaAs. Moreover, to avoid any stoichiometry loss due to dissociation of the adduct, the properties of organometallic molecules which feature a covalent bond between the group-III and group-V elements have also been investigated. These covalent compounds are probably formed in the MOCVD reactor when alkyl group-V compounds containing acidic hydrogen R_3?nMH_n (M = As, P; n = 1,2) are used. Such new precursors are also briefly reviewed.     

Growth of III–V semiconductors by molecular beam epitaxy and their properties

Advances in solid state device technology in the sixties established III–V materials as a new class of semiconductors for high speed microwave and highly efficient optical devices. Molecular beam epitaxy (MBE) is an extremely versatile thin film technique which can produce single-crystal layers with atomic dimensional controls and thus permit the preparation of novel structures and devices tailored to meet specific needs. Important factors to achieve high quality MBE growth such as in situ analysis, substrate preparations, growth conditions and layer properties are discussed.     

III–V nitrides: wurtzite symmetry and optical absorption

We report here on the study of the theoretical optical properties of Ga based nitrides systems. The dipolar matrix elements and hole effective masses are presented as a function of the reciprocal wavevector for strained bulk GaN. We compute then the A, B and C exciton binding energies for strained GaN/Al_0.2Ga_0.8N quantum structures and present the calculated optical density for a 50 Å wide well.     

Some aspects of the technology of III–V compound semiconducting layers

The electrical and electro-optic properties of the III–V compound semiconductors provide some important advantages obtained by the monolithic integration of light-emitting diodes or lasers with photosensors and high speed field effect transistors on the same semi-insulating substrates.     

Structural property of β-FeSi2 layers deposited on FeSi from a molten salt

The microstructural properties of the β-FeSi₂/FeSi structure prepared from a molten salt have been characterized using transmission electron microscopy (TEM). The β-FeSi₂ films were grown on FeSi substrates at the heat treatment temperature of 900 °C from 1 min to 24 h using the molten salt technique. It is found that the films consisted of a thin surface layer and a thick underlying layer with columnar-shaped domains. The crystallographic directions of the domains are mostly randomly oriented. The β-FeSi₂ domains in the film, however, have specific crystallographic orientation relationships with the adjoining domains and the FeSi substrate. A high density of the stacking faults on the β-FeSi₂ (100) planes was also observed through the films. Moreover, the growth evolution of the β-FeSi₂ domains, the defect characteristics and the formation mechanism of the defects are discussed.     

Structural and dielectric properties of (001) and (111)-oriented BaZr0.2Ti0.8O3 epitaxial thin films

We have grown and characterized BaZr_0.2Ti_0.8O₃ (BZT) epitaxial thin films deposited on (001) and (111)-oriented SrRuO₃-buffered SrTiO₃ substrates by pulsed laser deposition. Structural and morphological characterizations were performed using X-ray diffractometry and atomic force microscopy, respectively. A cube-on-cube epitaxial relationship was ascertained from the θ-2θ and φ diffractograms in both (001) and (111)-oriented films. The (001)-oriented films showed a smooth granular morphology, whereas the faceted pyramid-like crystallites of the (111)-oriented films led to a rough surface. The dielectric response of BZT at room temperature was measured along the growth direction. The films were found to be ferroelectric, although a well-saturated hysteresis loop was obtained only for the (001)-oriented films. High leakage currents were observed for the (111) orientation, likely associated to charge transport along the boundaries of its crystallites. The remanent polarization, coercive field, dielectric constant, and relative change of dielectric permittivity (tunability) of (111)-oriented BZT were higher than those of (001)-oriented BZT.     

X-ray investigations of III–V compounds: layers, nanostructures, surfaces

Some recent results of X-ray investigations of epitaxial layer systems are reviewed. The interest is directed to correlation phenomena in the distribution of misfit dislocations, to the coexistence of two phases of MnAs in a large range of temperatures, to the stress relaxation in quantum wire structures on InP and to the determination of phases of surface reflections.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Two-dimensional phase separation and surface-reconstruction driven atomic ordering in mixed III–V layers

In mixed III–V layers atomic species having different covalent tetrahedral radii are not distributed at random on their respective sublattices. Two types of deviation from randomness are observed: (i) phase separation, and (ii) atomic ordering. Phase separation is two-dimensional in nature, occurs on the surface while the layer is growing and is driven by surface thermodynamics. In contrast, atomic ordering is induced by subsurface stresses associated with (2×4) and (2×3) reconstructions of group V terminated (001) surfaces. These stresses bias the occupation of sites by atomic species differing in their tetrahedral radii and this feature leads to the evolution of double and triple period superlattices on ( 11)_B, (1 1)_B, and (111)_A, (11 )_A planes respectively.     

Structural characterization of basalt-based glass–ceramic coatings

There are a lot of technologically interesting characteristics of glass–ceramics, which are hard, wear resistant, oxidation and corrosion resistant ceramic materials. In the present study, the production of the basalt-based glass–ceramic coating by atmospheric plasma spray technique and their structural characterization were reported. Basalt-based glass coating was performed on AISI 1040 steel substrate which was pre-coated with Ni–5 wt% Al by using plasma spray gun. Basalt coatings of the glass form were crystallized at 800, 900 and 1000 °C for 1–4 h in orders to transform to the glass–ceramic structure. The presence of augite [(CaFeMg)SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂] and aluminian diopside [Ca(Mg,Al)(Si,Al)₂O₆] crystalline phases formed in the basalt-based glass–ceramic coating layer was detected by X-ray diffraction analysis. Optical microscopy with micrometer was used for metallographic examinations. Differential scanning calorimeter was used for determining the crystallization temperature of glass form basalt-based coatings. Microhardness measurements were carried out on the basalt-based glass–ceramic coating layer with Vickers indenter. The hardness of coating layers is changing between 1009 and 1295 HV_0.05 depending on crystallization temperature and process times. It was found that, the higher the crystallization temperature, the more the crystalline phases were resulted. In addition, the lower the crystallization temperature and the longer the treatment time, the harder the coating layer became.     

Structural models for epitaxy of hexagonal phases of boron nitride on Si(001)

Interface models for matching either w-BN or h-BN layers to an Si(001) substrate, are considered. Aligning the [2 − 1 − 10] direction of h-BN (or equivalently the [−1 − 120] direction in w-BN) along [1 − 10] the Si(001), chemical saturation of interface bonds can be achieved. For h-BN, orientations of the basal plane parallel and perpendicular to the surface are both considered. The total energy of atomic clusters constructed to represent these models, are calculated using the AM1 quantum chemical Hamiltonian, and atomic geometries optimised allowing full symmetric relaxation of interface atoms. Only the perpendicular h-BN basal plane model is found to be energetically favourable compared to earlier calculations for cubic boron nitride.     

Structural investigations of homoepitaxial Si films grown at low temperature by pulsed magnetron sputtering on Si(111) substrates

Using pulsed magnetron sputtering at low substrate temperature (T_s = 580 °C) the homoepitaxial growth on Si(111) was studied. The films were comprehensively characterized by cross-section transmission electron microscopy and various diffraction methods. Up to a film thickness of 1240 nm no breakdown of the epitaxial growth was observed. The surface microstructure, characterized by electron backscatter diffraction, exhibits exclusively crystalline structure with (111) orientation. Careful analysis of selected area electron diffraction patterns and high-resolution X-ray diffraction data clearly proves the existence of twinning/stacking faults in the {111} planes. Besides these defects – which are typical for low-temperature epitaxy – no additional significant defects related to the energetic particle bombardment by the sputter deposition method are observed.     

Growth of InN layers on Si (111) using ultra thin silicon nitride buffer layer by NPA-MBE

Indium nitride (InN) epilayers have been successfully grown by nitrogen-plasma-assisted molecular beam epitaxy (NPA-MBE) on Si (111) substrates using different buffer layers. Growth of a (0001)-oriented single crystalline wurtzite-InN layer was confirmed by high resolution X-ray diffraction (HRXRD). The Raman studies show the high crystalline quality and the wurtzite lattice structure of InN films on the Si substrate using different buffer layers and the InN/β-Si₃N₄ double buffer layer achieves minimum FWHM of E₂ (high) mode. The energy gap of InN films was determined by optical absorption measurement and found to be in the range of ~ 0.73-0.78 eV with a direct band nature. It is found that a double-buffer technique (InN/β-Si₃N₄) insures improved crystallinity, smooth surface and good optical properties.     

Optimization of ZnSe-SiO2 nanostructures deposited by radio-frequency magnetron sputtering: Correlations between plasma species and thin film composition, structural and microstructural properties

ZnSe nanoparticle doped SiO₂ films have been grown on various substrates at different deposition temperatures, radio-frequency power, Argon pressures and substrate to target distances, by means of reactive magnetron sputtering. A detailed study of the correlations between plasma species and thin film composition, structure and morphology is investigated using X-ray reflectivity and diffraction, Raman and optical emission spectroscopies and Rutherford backscattering technique. It is evidenced that the most sensitive species in the plasma is the Selenium and that the optimal deposition parameters correspond to random stress-free films with a high content of quasi-stoechiometric ZnSe cubic nanocrystallites. A few amount of ZnSe in the hexagonal structure is also evidenced in these films. Using proper deposition parameters, the SiO₂/ZnSe proportion in the films and the mean ZnSe particles size around 3 nm are easily monitored.     

Twinning of YBa2Cu3O7 thin films on different substrates

The twinning of YBa₂Cu₃O₇ (YBCO) thin films with c-axis orientation on (001) MgO, (001) SrTiO₃, (012) LaAlO₃, (110) NdGaO₃ and (001) NdGaO₃ substrates, prepared by laser ablation, has been examined using a combination of and θ/2θ scans at a four-circle diffractometer. On all substrates, except for (001) NdGaO₃, the tetragonal to orthorhombal phase transition results in four different orientations of YBCO twins relating to the substrate. On (001) NdGaO₃ only two different twin orientations, accompanied by a slight lattice monoclinization, has been observed.     

Epitaxial and polycrystalline BaFe12O19 thin films grown by chemical vapour deposition

Epitaxial and polycrystalline barium hexaferrite BaFe₁₂O₁₉ thin films were prepared by metalorganic chemical vapour deposition (MOCVD). Films were grown by a liquid MOCVD technique which aim is to control precisely the precursor vapour pressures. Two kinds of substrates were used: sapphire (001) and silicon thermally oxidized. On Si/SiO₂ films are polycrystalline and the magnetization is isotropic. On Al₂O₃ (001), structural studies reveal the films to be predominantly single phase, well crystallized without annealing procedure and with the c-axis perpendicular to the film plane; epitaxial relationships between the film and the substrate were determined. The magnetic parameters, deduced from vibrating sample magnetometer measurements, show a high dependence of the magnetization with the orientation of the field with respect to the surface of the film.     

Dielectric and structural characterization of KNbO3 ferroelectric thin films epitaxially grown by pulsed laser deposition on Nb doped SrTiO3

KNbO₃ thin films were deposited on SrTiO₃ substrates by pulsed laser deposition. The X-ray diffraction patterns highlight an epitaxial growth according to the (011) orientation. This epitaxial growth was then confirmed by Electron Channeling Pattern. In agreement with the structural characteristics the dense microstructure consists in regular and ordered grains. Dielectric measurements were performed in the 20 Hz to 1 MHz frequency range on a KNbO₃ thin film grown on 2 at.% Nb doped (100)SrTiO₃ substrate in a large range of temperature in order to investigate the paraelectric-ferroelectric transition. Measurements at room temperature revealed a dielectric constant of 450 at 10 kHz and a minimum value of the loss tangent of 0.075 at 100 kHz. Dielectric study in the 20-600 °C temperature range showed a maximum of permittivity at the Curie temperature T_c = 410 °C and evidenced a “progressive” first-order phase transition, different from the classical “diffuse” transition.     

Epitaxial bilayered Nb–Mo(001) films: growth, characterization and size effect in electron conductivity

Mo–Nb and Nb–Mo epitaxial thin (10–200 nm) films growth on the r- sapphire plane under ultra high vacuum by laser ablation deposition at the growth temperature 750°C were tested. Grown films were characterized by reflected high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning atomic force microscopy and electron transport measurements. It was found that bilayered films are high quality heteroepitaxial single-crystalline (001) films with low surface roughness (0.3–0.5 nm) and large residual electron mean free path, compared to the film thickness. The superconducting transition temperature of Nb–Mo films with equal layer thickness varies from 5 up to 9 K as the thickness increases from 10 to 100 nm. Physical properties of the films are close to each other both for Nb/Mo and Mo/Nb films of equal layer thickness and do not depend on the alternation of Mo and Nb layers. This suggests the same film structure quality and mutual epitaxy of the metals. The size dependence of electron conductivity of bilayered Mo–Nb films showed that it is determined by the dimension of individual layers, constituting the film, where the effect of ‘contact’ potential formed at the interface between Mo and Nb layers and a fluctuation of film bending is important.